1. Field of the Invention
The present invention relates to a process for preparing high-temperature superconducting thin films, more particularly to a process for preparing thin films of high-temperature oxide superconductors possessing improved crystallinity and electrical properties on a substrate.
The term high-temperature oxide superconductor means new type oxide superconductors possessing the critical temperature (Tc) of higher than 30 K such as Y-Ba-Cu oxide, Bi-Sr-Ca-Cu oxide, Ti-Ba-Ca-Cu oxide or the like.
2. Description of the Related Art
These new type high-temperature superconductors are expected to be utilized to fabricate electronics devices such as Josephson element, superconducting transistors or the like. In order to realize these electronics devices, it is indispensable to prepare their thin films.
It is known that these high-temperature oxide superconductors show anisotropy in their superconducting properties. In fact, their crystals show higher critical current density (Jc) along a direction which is perpendicular to c-axis than the other directions, namely a-axis and b-axis. Therefore, a variety of studies have been made to prepare c-axis oriented thin films of the oxide superconductors in which electric current flows mainly in parallel with a surface of the thin film. Today, such c-axis oriented single crystal thin films of high quality can be prepared by sputtering, vacuum-evaporation, laser abrasion technique or the like on single crystal substrates such as SrTiO.sub.3 and MgO.
The high-temperature superconductors exhibit anisotropy in their coherent length also. Namely, the coherent length along the a-axis is longer than that of along the c-axis. In fact, the coherent length along the c-axis is several angstroms (.ANG.) while the coherent length along the a-axis is several ten angstroms (10 .ANG.). In certain applications of oxide superconductors such as electronics devices, for example Josephson Junctions, the a-axis oriented thin films possessing such longer coherent length are requested.
The "a-axis oriented thin film" means thin films made of crystal whose a-axis is substantially perpendicular to a surface of the thin film.
It is known that such a-axis oriented thin films of oxide superconductors can be prepared or realized, for example by sputtering, under such a condition that a substrate on which the thin film is deposited is heated at a temperature which is lower by about 10.degree. C. than a temperature which is requested to realize the c-axis oriented thin film. The crystallinity of the resulting thin film of oxide superconductor, however, is spoiled when the substrate temperature is lowered because oxygen supply into the crystal becomes insufficient. Therefore, the a-axis oriented thin films of oxide superconductors prepared by methods known in the prior art show relatively poor electrical properties.
In the case of techniques other than sputtering, it is difficult to prepare the a-axis oriented thin films of oxide superconductors directly on a substrate even if the substrate temperature is controlled or lowered. Particularly, it is very difficult to prepare the a-axis oriented thin films of oxide superconductors directly on a substrate by vacuum-evaporation, although the vacuum-evaporation technique has many advantages, including that a composition in the thin film can be controlled easily and a thin film of large area can be prepared at relatively higher deposition rate.
The inventors of the present application proposed, in a copending U.S. Pat. application Ser. No. 07/668,263 now U.S. Pat. No. 5,151,408, a process for preparing an axis oriented thin film of oxide superconductor possessing improved electrical properties by vacuum-evaporation technique by interposing another a-axis oriented intermediate layer between the a-axis oriented thin film and a substrate.
Now, the inventors of the present application found that the same technique as is disclosed in the U.S. Pat. application Ser. No. 07/668,263 is applicable to prepare an a-axis oriented thin film of oxide superconductor by the laser evaporation method which has advantages that thin film can be prepared rapidly in the absence of electromagnetic field and that deposition conditions can be controlled easily compared to the other techniques.
Although the laser evaporation method itself is known and is used for preparing thin films of superconducting oxides, it has been difficult to prepare the a-axis oriented thin films of oxide superconductors directly on a substrate by the laser evaporation method because of the same reason as above.
Therefore, an object of the present invention is to provide a process for preparing the a-axis oriented thin films of oxide superconductors possessing improved electrical properties by the laser evaporation method.